Can exercise wear out joints ?
Usually not; in most cases, exercise does more good than harm to joints, 
says Professor Patrick McNeil, a rheumatologist and the president of Arthritis Australia.  
1.       Movement is good for our joints.
That’s because cartilage in our joints does not have arteries that deliver bloodInstead it relies on movement of the joint to create a pumping action that circulates fluid containing oxygen and nutrients.   
2.       Sore joints are a clear sign of osteoarthritis.
It is increasingly common as we age. From there , people jump to the conclusion that these aches result from a lifetime of activity. That assumes that with every movement our limbs make, we are literally wearing our joints away. But is this really the case ? 
No, says Professor Patrick McNeil. 
3.       Our joints do not have a use-by date.
"I think it's a myth to make the general statement that exercise is bad for your joints or actually wears your joints out. There's no evidence for that." McNeil also says plenty of older people who've been active all their lives never develop osteoarthritis. And he addsthat the idea our joints are like car tyres or light bulbs with a limited number of "uses" before their lifetime expires is simply untrue. 
4.       For most people, exercise helps joints stay healthy.
The following advice is from the Mayo Clinic:
“Exercise is crucial for people with arthritis. It increases strength and flexibility, reduces joint pain, and helps combat fatigue. … Even moderate exercise can ease your pain and help you maintain a healthy weight. When arthritis threatens to immobilize you, exercise keeps you moving. … Lack of exercise actually can make your joints even more painful and stiff. That's because keeping your muscles and surrounding tissue strong is crucial to maintaining support for your bones.Not exercising weakens those supporting muscles, creating more stress on your joints.”  Obviously, this general guide must be subject to one caution: change any movement the moment you feel pain at the joints. 
5.       For some people, exercise for joints could be harmful   
McNeil says. When there is a misaligned joint – perhaps because of weakened muscles or because you were born with unusually shaped joints – it's feasible that movement might have some role in wearing cartilage away. But a person may also need to have a genetic predisposition for this to happen. "You could be born with cartilage that will last, no matter what you do to it. Or you could be born with cartilage that's less durable." And even if your joints are perfectly aligned, this can change after an injury.
6.       Sports that expose joints to extreme forces make injuries more likely.
For example, Taekwondo, soccer, Aussie football and netball can be like that. They are known to raise the odds of joint trouble down the track. For instance, tears to one of the four major ligaments of the human knee (anterior cruciate ligament) (ACL) – a major ligament in the knee – carry a 70 per cent risk of osteoarthritis within 10 to 15 years. Even when the ligament is surgically repaired, the initial injury changes the stability of the knee permanently, so that over time it's more likely cartilage will break down.
7.       "Exercise doesn't seem to be relevant at all to osteoarthritis of the hands,"
McNeil says. The processes leading to cartilage loss may be different in different parts of the body. Osteoarthritis of the hands for instance seems to have a very strong genetic basis. "If use was a factor, you'd expect right-handed people to have more arthritis in their right hand than in their left. But that isn't the case. "We still don't understand all the triggers." The bottom line is that the evidence so far doesn't suggest a strong connection between simple repetitive use of a joint and the development of osteoarthritis. So if your joints aren't bothered by the exercise you're doing, it's unlikely you're doing them harm, McNeil says. But if you have already been diagnosed with joint damage, or have pain that suggests it, it's possible the wrong sort of exercise could make your arthritis worse.
8.       High impact exercise is bad for people with arthritic problems.
"If you have osteoarthritis in weight-bearing joints such as the hips, knees and lower spine for instance, high impact exercise like running can certainly aggravate symptoms and it's probable it might accelerate progression [of the damage to the joints], although I think that's still an open question," McNeil says. "In those cases, I'd probably recommend a lower impact exercise such as swimming or bike riding that puts less stress on the affected joints."
9.       Exercise is actually one of the best treatments for arthritis.                           
McNeil says exercise also: decreases pain, helps maintain mobility and flexibility of joints and improves muscle strength which helps hold joints in their correct alignment and takes pressure off sore spots. It also helps maintain a healthy weight. That’s important because excess weight increases the risk of osteoarthritis; it stresses joints. "It's much better to be physically active than to hold back because of your joints," says McNeil.
10.   Exercise – like tai chi – helps to reduce risk of falls.                            
Oldies tend to fall. That can lead to a more sedentary lifestyle and even disability. To reduce that risk, consider an exercise in slow motion like tai chi. It can to strengthen legs and give a better sense of stability of movement.
How much physical activity do older adults need ? 
If you're 65 years or more, generally fit, and have no limiting health conditions, you need 2 hours and 30 minutes (150 minutes) ofmoderate-intensity aerobic activity (i.e., brisk walking) every week and muscle-strengthening activities on 2 or more days a week that work all major muscle groups (legs, hips, back, abdomen, chest, shoulders, and arms).